Vaccines against bacterial and viral diseases have played an important role in reducing infectious diseases in humans; however, there is still a need for innovative vaccines to reduce the current global burden of infectious diseases. Cold-adapted viruses have been used for decades as vaccines against human viral diseases. The best known example of such a vaccine is the Sabin polio virus vaccine. An alternate example is a cold adapted influenza vaccine called FluMist® (Medimmune LLC, Gaithersburg, Md., USA), which was introduced in the U.S. in 2003. FluMist® has been shown to be considerably more effective in certain demographic groups than influenza vaccines that practice the more common vaccination strategy of using inactivated virus to stimulate an immune response. Typically cold-adapted or “temperature-sensitive” (TS) viral strains have been developed by passing the virus repeatedly in eggs or cell culture at low temperatures and then testing the progeny for their inability to grow above about 37° C., generally thought of as the “normal” human body temperature.
The concept of a “normal” human body temperature takes into consideration anatomical sites, individual variations, gender, physiological conditions and ambient temperature. Despite the number of variables, the human body can function only in a very narrow temperature range, which is generally about 36° C.-39° C. If the human body core temperature falls to about 35° C., the body must be warmed or death will ensue. The skin temperature is always cooler than the body core regardless of the ambient temperature and clothing worn. At moderate temperatures (e.g., 21° C.), the temperature of the skin is about 32° C.-35° C.
Those skilled in these arts are of the view that bacteria generally have a set of about 100 to 150 genes, called “essential genes” that are absolutely required for maintenance of bacterial viability. Identifying essential genes is difficult due to their nature, as knockouts of these genes results in death of the organism. Essential genes encode proteins composed of amino acid sequences that are highly conserved among almost all bacterial genera and species. This conservation presumably reflects their common function and structure among the different species. A select number of essential genes have been shown to be competent in substituting for a homologue in another bacterial species and in some cases these substitutions were from distantly related bacterial species. The conservation of amino acid sequences is widespread among bacteria, the deduced amino acid sequences of essential genes from psychrophiles and thermophiles shows high identity with their mesophilic counterparts. Microbiologists have generally used conditional lethal mutations, such as TS mutations, to identify essential genes.
Many bacterial species play significant roles in the global burden of infectious diseases. However, the causative agent of tuberculosis is probably the most significant contributor to human morbidity and mortality caused by an infectious bacterial disease. Although the Bacille Calmette-Guérin (BCG) vaccine has been used for several decades to protect against tuberculosis, its low efficacy has failed to lower the incidence of tuberculosis to acceptable levels.